The present invention relates to micromechanical resonators. In particular, the present invention is related to internal electrostatic transduction structures for bulk-mode resonators.
Surface micromachining technology supports fabrication of multifrequency, electrostatically transduced bulk resonators on a single substrate. Using lithography, a single mask can include multi-frequency filters, oscillators and mixers if the microstructures are excited in lateral modes (i.e., modes within the plane of the substrate). Electrostatic excitation of microresonators has been problematic due the relatively large motional resistance of the device, which directly results from the low transduction efficiency compared with piezoelectric transduction (e.g., see, Humad et al, “High Frequency Micromechanical Piezo-on-Silicon Block Resonators,” IEDM, 957-960 (2003)). For lateral bulk acoustic microresonators, the reduced transducer area further increases the motional resistance (e.g., see, Mattila et al., “Micromechanical Bulk Acoustic Wave Resonator,” Ultrasonics Symposium, 945-948 (2002)). Several approaches to increasing transducer area include forming a coupled array of resonators (e.g., see, Demirci et al., “Mechanically Corner-Coupled Square Microresonator Array for Reduced Series Motional Resistance,” Transducers, 955-958 (2003)) and large diameter bulk annular ring resonators (e.g., see, Li et al., “Micromechanical “Hollow-Disk” Ring Resonators,” MEMS, 821-824 (2004)). However, to reach motional resistances on the order of 50 Ω, a coupled array of 100 resonators or a 400 μm diameter ring resonator would be needed. The signal routing challenges for these structures will be daunting at GHz frequencies and the chip area occupied by these resonator designs will be larger than for a Film Bulk Acoustic Resonator, or an FBAR, which has motional resistance of 2 Ω.
There is therefore a need for an improved electrostatic transducer with enhanced efficiency that does not suffer from the design disadvantages of either air-gap electrostatic or piezoelectric transducers.